1. Field of the Invention
The present invention relates to an electron gun in a color CRT (Cathode Ray Tube), in which a distance between a central electron beam and an outer electron beam is made greater on a deflection center plane for improving a resolution.
2. Background of the Related Art
FIG. 1 illustrates a horizontal longitudinal section of a related art cathode ray tube.
Referring to FIG. 1, the related art cathode ray tube is provided with a panel 1 and a funnel 2 of a front part and a rear part of the cathode ray tube, a neck part 2a at an end of the funnel 2, an electron gun 3 in the neck part 2a for emitting R, G, B electron beams 3a, a deflection yoke 4 on an outer circumference of the funnel 2 for deflection of the electron beams in an upper, lower, left, of right direction, a color purity magnet 5 in front of the deflection yoke 4 for fining tuning a path of the electron beams 3a, a shadow mask 6 fitted between the electron gun 3 and the panel 1 for selective pass of the deflected electron beams 3a, and a fluorescent surface 7 of R, G, B fluorescent materials on an inside surface of the panel 1.
FIG. 2 illustrates a partial longitudinal section of an electron gun built in a neck part of a color CRT in FIG. 1.
Referring to FIG. 2, the electron gun 3 is provided with cathodes 8, a control electrode 9, an acceleration electrode 10, first and second pre-focusing electrodes 11a and 11b, a focusing electrode 12 and an anode 13, in an order thereof, so that a preset voltage is applied to each of the electrodes.
Upon putting the cathode ray tube into operation, electron beams 3a are emitted from the cathodes 8, controlled, accelerated, and pre-focused by the control electrode 9, the acceleration electrode 10, and the first and second pre-focusing electrodes 11 (11a and 11b), and subjected to main focusing by a main focusing electro-static lens formed between the focusing electrode 12 and the anode 13 of a potential difference. Then, the electron beams 3a are deflected in an upper, lower, left, or right direction by the deflection yoke 4, pass through a shadow mask 6 selectively, and land on a fluorescent surface, to form a picture on the panel 1. A color purity of the picture formed thus may be adjusted more precisely as worker adjusts the color purity magnet 5 to change a path of the electron beams.
In the meantime, it is known that a picture quality becomes the better as a spot size of the landed electron beams 3a is made the smaller. The spot size of the electron beams 3a is proportional to a diameter of the main focusing electrostatic lens, and the size of the main focusing electrostatic lens is proportional to sizes of electron beam pass through holes 12a and 13a formed in parts opposite to the focusing electrode 12 and the anode 13.
FIG. 3 illustrates a perspective view of the focusing electrode and the anode of the electron gun in FIG. 2.
Referring to FIG. 3, in order to form a large diametered main focusing electrostatic lens, there are horizontally elongated track type rims 12b and 13b each forming single electron pass through hole 12a or 13a for passing the three electron beams 3a in parts opposite to the focusing electrode 12 and the anode 13 respectively, and electrostatic field controlling bodies 14 and 15 each provided at a point moved a distance back from the rim 12b or 13b, respectively.
The horizontally elongated track type rim 12b or 13b has a small height and a great width, permitting an electric field to penetrate shallow in a vertical direction and deep in a horizontal direction, forming a large equipotential surface curvature in the vertical direction, and a small equipotential surface curvature in the horizontal direction. According to this, a single horizontally elongated main focusing electrostatic lens focuses the electron beams 3a strongly in the vertical direction, and weakly in the horizontal direction.
However, the electrostatic field controlling bodies 14 and 15 suppress a horizontal field penetration, resulting to form the curvature of horizontal equipotential surface larger. Consequently, a horizontal focusing power of the main focusing electrostatic lens becomes stronger, making the horizontal and vertical focusing powers of the main focusing electrostatic lens the same.
FIGS. 4A-4D illustrate various examples of electrostatic field controlling bodies fitted to the focusing electrode and the accelerating electrode in FIG. 3. FIG. 4A illustrates a front view of an LB (large aperture with blade) type electrostatic field controlling body disclosed in U.S. Pat. No. 5,512,797 by the inventor. The LB type electrostatic field controlling body 14 or 15 is provided with a rectangular electron beam pass through hole 14G or 15G at a center, and vertical blades 14a and 15a at both sides thereof. The blades 14a and 15a make a section modulus greater, to reinforce the electrostatic field controlling bodies 14 and 15 against deformation. However, the blades 14a and 15a interfere horizontal penetration of an electric field, resulting to form a greater horizontal curvature of the main focusing electrostatic lens, that focuses the electron beams 3a excessively in the horizontal direction.
FIG. 4B illustrates a front view of an EA (elliptical aperture) type electrostatic field controlling body developed by Hitachi. The EA type electrostatic field controlling body 14 or 15 is a plate having a vertically elongated elliptical central electron beam pass through hole 14G or 15G, and vertically elongated semi-elliptical outer electron beam pass through holes 14R and 14B, or 15R and 15B. Since the electrostatic field controlling body is not provided with the blades 14a and 15a as shown in FIG. 4A, the horizontal penetration of the electric field in not interfered, to reduce the horizontal curvature of the main focusing electrostatic lens, that permits to form a large sized main focusing electrostatic lens having vertical and horizontal direction harmonized. However, since the electrostatic field controlling body is not provided with the blades 14a and 15a, the electrostatic field controlling body has a smaller section modulus, and is liable to deform.
FIG. 4C illustrates a front view of an AEA (Advanced Elliptical Aperture) type electrostatic controlling body disclosed in U.S. Pat. No. 5,146,133 by Hitachi. The AEA type electrostatic controlling body 14 is a plate having three, in line, vertically elongated elliptical electron beam pass through holes 14R, 14G, and 14B, fitted inside of the focusing electrode 12. It is known that the AEA type electrostatic controlling body 14 prevents unbalance between the outer electron beams R, and B and the central electron beam G when the electrostatic controlling body 14 is placed away from the rim 12b, i.e., near to the second pre-focusing electrode for making a size of the main focusing electrostatic lens greater.
FIG. 4D illustrates a front view of an XL (Extended Large Aperture) type electrostatic controlling body developed by RCA. The XL type electrostatic controlling body 14 or 15 is a plate having the circular three in-line electron beam pass through holes 14R, 14G, and 14B, or 15R, 15G, and 15B. It is known that it is difficult to form spot sizes of the central electron beam G and the outer electron beam R, or B are the same.
In the meantime, FIG. 5 illustrates an exemplary process in which the electron beams are deflected at the deflection center plane, pass through a shadow mask, and land on the fluorescent surface, schematically. In this instance, the shorter the distance xe2x80x98Qxe2x80x99 between the fluorescent surface 7 and the shadow mask 6, the less the mis-landing of the electron beams 3a (R, G, and B) on the fluorescent surface 7 caused by deformation of the shadow mask 6 coming from thermal expansion or vibration. Therefore, in fabrication of the CRT, the distance xe2x80x98Qxe2x80x99 between the fluorescent surface 7 and the shadow mask 6 is set to be minimum. The xe2x80x98Qxe2x80x99 can be made minimum by varying variables on the right side of an equation shown below.
Q=Phxc3x97L/3S2
1. A distance xe2x80x98Lxe2x80x99 between the shadow mask 6 and the deflection center plane 4a. 
2. A horizontal distance Ph between centers of the electron beam pass through holes 6a. 
3. A distance S2 between center axes of the R, G, and B electron beams 3a at the deflection center plane.
The xe2x80x98Lxe2x80x99 is set to be minimum, and the xe2x80x98Phxe2x80x99 is set to be minimum as far as productivity is the greatest. At the end, what is left in above equation for minimizing xe2x80x98Qxe2x80x99 is S2.
As shown in FIGS. 6A and 7, S2 varies with the following variables.
1. A distance S0 between centers of the central electron beam xe2x80x98Gxe2x80x99 and the center of the outer electron beam xe2x80x98Rxe2x80x99, or xe2x80x98Bxe2x80x99 from the cathodes to the acceleration electrode 10.
2. A distance S1 between centers of the central electron beam pass through hole 11G and the outer electron beam pass through hole 11R, or 11B of the first pre-focusing electrode 11a. 
3. A distance xe2x80x98Pxe2x80x99 between centers of the central main focusing electrostatic lens and the outer main focusing electrostatic lens.
The S0 is the same with a distance between centers of the central cathode 8G and an outer cathode 8R, or 8B, centers of the central electron beam pass through hole and an outer electron beam pass through hole of the controlling electrode 9, or centers of the central electron beam pass through hole 10G and an outer electron beam pass through hole 10R, or 10B of the accelerating electrode 10.
The xe2x80x98S1xe2x80x99 is set such that an eccentricity xe2x80x98S1/S0xe2x80x99 of the xe2x80x98S1xe2x80x99 to the related art xe2x80x98S0xe2x80x99 is to be equal to, or greater than unity xe2x80x981xe2x89xa6S1/S0xe2x80x99, for converging the outer electron beams R, and B toward the central electron beam G. The outer electron beams R, and B are converged toward the central electron beam G according to the following process. In general, the first pre-focusing electrode 11a has a voltage higher than the accelerating electrode 10 applied thereto, and the electron beams 3a moves from a low voltage to a high voltage. Therefore, as shown in FIG. 6A, when S1 is greater than S0, since the outer electron beams R and B passed through the outer electron beam pass through holes 10R, and 10B of the accelerating electrode 10 respectively go closer to the bridges xe2x80x98Bxe2x80x99 surrounding outer sides of the central electron beam R, and B pass through holes respectively, the outer electron beams R and B are converged toward the central electron beam G.
In the foregoing electron gun, if it is intended to make the S2 greater, i.e., the xe2x80x98Qxe2x80x99 smaller, for improving a picture quality of the CRT, it is required to change S0, S1, and P significantly on the whole, accompanying entire change of the electron gun, that imposes a limitation in improvement of the picture quality, as an inside diameter of the neck part the electron gun is fitted therein is limited.
Accordingly, the present invention is directed to an electron gun in a color CRT that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an electron gun in a color CRT, in which a distance S2 between a central electron beam G and an outer electron beam R, and B is made greater at a deflection center plane for improving a resolution of a picture.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the electron gun in a color CRT includes a controlling electrode and an accelerating electrode each having a distance S0 between a central electron beam pass through hole and an outer electron beam pass through hole for passing electron beams emitted from cathodes, a pre-focusing electrode having a distance S1 between a central electron beam pass through hole and an outer electron beam pass through hole, a focusing electrode and an anode each having a rim at an opposite part for forming single electron beam pass through hole and an electrostatic field controlling body inside of the rim, for forming a large sized main focusing electrostatic lens by a potential difference, and diverging means for diverging the outer electron beams incident on the focusing electrode from the pre-focusing electrode outwardly with respect to the central electron beam, wherein a ratio WS/H of a sum WS of a horizontal diameter of the central electron beam pass through hole of the electrostatic field controlling body and a minimum width xe2x80x98txe2x80x99 of the bridge xe2x80x98Bxe2x80x99 surrounding horizontal direction outsides of the central electron beam pass through hole to a horizontal width xe2x80x98Hxe2x80x99 of the rim is set to be 0.31xe2x89xa6WS/Hxe2x89xa60.34 for shifting positions of the outer main focusing electrostatic lenses outward in correspondence to the divergence of the outer electron beams with respect to the central main focusing electrostatic lens.
The diverging means is one S1/S0 is set to be in a range of 0.96xe2x89xa6S1/S0xe2x89xa61.
The diverging means is a color purity magnet fitted to a deflection yoke of the color cathode ray tube.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.